Size reduction stressing mechanisms

Size reduction is an extremely important unit operation, whereby materials are subjected to stress in order to reduce the size of individual pieces. The stress is apphed by transmitting mechanical force to the soHd. 

Fig. 3. Stressing mechanisms (a) single particles or (b) a bed of particles cmshed between two solid surfaces impact of a particle against (c) a solid surface or (d) another particle (e) cutting (f) shearing forces or pressure waves and (g) plasma reaction, an example of size reduction by nonmechanical energy.

FIG. 20-20 Stressing mechanisms to cause size reduction. [Rumpf, Chem. Eng. Tech., 37(3), 187-202 (1965).]... 

The melt cools down when glass fibers are added. It is therefore necessary that the melt temperature is sufficiently high before adding the fibers to ensure that the polymers do not cool down to their solidification temperature. Too great an increase in viscosity caused by cooling will prevent the melt from wetting the fibers, in which case the melt will cause mechanical stress and damage to the fibers. The size reduction of the fibers depends on the... 

Introduction. In size reduction of solids, feed materials of solid are reduced to a smaller size by mechanical action. The materials are fractured. The particles of feed are first distorted and strained by the action of the size-reduction machine. This work to strain the particles is first stored temporarily in the solid as strain energy. As additional force is added to the stressed particles, the strain energy exceeds a certain level, and the material fractures into smaller pieces. 

In addition to particle size reduction, roller compaction and tableting are other downstream processes that will likely to be impacted by the mechanical properties of the milled extrudate. In the case of tableting, the extrudate may be subjected to localized high stresses which can induce particle breakage, elastic deformation, and/or plastic flow that affect compactability and tablet hardness. 

Amorphous solid dispersions are prepared primarily with amorphous and/or semicrystalline materials, and therefore, the mechanical behavior of the extrudate is generally viscoelastic in nature. The materials viscoelasticity implies a strain-rate dependence of the mechanical response and time-dependent mechanical behavior such as creep and stress relaxation. For example, in cases of high strain rates, these materials tend to be more brittle than under slower strain rates where viscous flow and other molecular relaxations can dissipate the energy without fracture. Thus, high strain rates are beneficial for particle size reduction operations. 

It is possible to identify three stresses mechanisms responsible for particle size reduction in mills ... 

Although a notable reduction of the phase size is reported for such compatibilized blends [64-67], the overall mechanical toughness remains unsatisfactorily low [64], As one reason, thermal stresses at the interface between PPE and SAN, occurring as a result of the different thermal coefficients of expansion during solidification following melt-processing, are identified as a crucial reason for the observed brittle behavior [68],... 

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